> I'm using a record like this
>
> {
> "type" : "record",
> "name" : "TestPacket",
> "fields" : [
> { "name" : "Id" , "type" : "int"},
> { "name" : "Value" , "type" : ["string","null"]}
> ]
> }
>
> But if I just not write anything to the 'Value' filed,
> avro_writer_memory() refuses to validate it. How do you normally use the
> optional fields with the C-API?
Hi Markus
In Avro, an “optional” field is really just a common pattern where you've got a union,
with a “null” for one branch, and some other type for the second branch. And with all
union types, you have to explicitly tell the Avro library which branch is active for a particular
value. (Think of the union as a container that must always contain exactly one value, but
where the type of that value can change.)
So, if you're using the new value API, you'd use something like:
avro_value_t rec = /* given */;
avro_value_t field, branch;
avro_value_get_by_name(rec, "Value", &field, NULL);
avro_value_set_branch(&field, 1, &branch);
avro_value_set_null(&branch);
and if you're using the old datum API:
avro_datum_t rec = /* given */;
avro_datum_t field, branch;
avro_record_get(rec, "Value", &field);
avro_union_set_discriminant(field, 1, &branch);
/* no avro_null_set in the old API */
In set_branch/set_discriminant, the 1 refers to the index of the "null" type in the union.
I tend to use branch 0 for the null type in these kinds of unions, but that's just a matter
of aesthetics.
cheers
–doug